In the field of magnetic discs, there is a so-called rigid magnetic disc and as a base plate for the magnetic disc, a rigid material having less flexibility is used. For example, an aluminum plate (e.g. JIS A5086) is usually used as a base plate for the rigid magnetic disc.
Such a rigid magnetic disc is ordinarily produced by grinding an aluminum plate using a grinding machine to reduce spacing between the surface of the aluminum plate and a magnetic head in the case of using the aluminum plate for magnetic disc so that high-density recording can be performed and thereafter forming thereon a magnetic layer by vapor deposition, spin coating, etc. In this case, the surface of the aluminum plate is as smooth or flat as possible for enabling high-density recording or reproducing but it has hitherto been difficult to obtain a surface of less than 0.1 .mu.m in a center line mean roughness Ra in the case a magnetic disc using a conventional aluminum base plate. Furthermore, since an aluminum base plate is lacking in flexibility, the web path, etc., are limited in the case of forming a magnetic layer on such an aluminum base plate, which makes the handling thereof greatly troublesome.
Moreover, high-density magnetic recording is greatly influenced by dust attached to the surface of a magnetic disc and hence it is required to make a magnetic disc with no such dust on the surface thereof. This makes the difficult production step of magnetic discs more troublesome and complicated as well as requiring large equipment costs.
On the other hand, since a conventional base plate, such as an aluminum plate, is rigid and has less flexibility, in the case of tracing a magnetic layer of a magnetic disc using such a base plate with a magnetic head, it is very difficult to maintain tracing with narrow spacing between the surface of the magnetic disc and the magnetic head, wherein errors in signal frequently occur.
Furthermore, when such a magnetic disc is traced with a magnetic head with narrow spacing, the sliding friction force is liable to be concentrate on the surface of the magnetic layer in contact with the magnetic head. Thus, since the base plate is rigid, surface breakage often occurs which shortens the life of the magnetic disc.
Still further, there is also a disadvantage that the ground aluminum base plate itself as described above is expensive.
On the other hand, recently, as shown in FIG. 1, a magnetic disc has been prepared by forming a concave portion on each of the surfaces of a disc base plate 1 and attaching a floppy disc or flexible disc having a magnetic layer on one surface thereof (hereinafter, such a disc having a magnetic layer is referred to as flexible disc) to each of the surfaces of the base plate 1 with the magnetic layer on the outside to provide a gap 3 between the back surface of the flexible disc 2 and the base plate 1.
The magnetic disc of this type is being watched with keen interest since the technique of flexible disc is applied as it is and a magnetic layer having smooth surface and high durability is used as the magnetic layers for the magnetic disc, which can eliminate the above-described disadvantages of a conventional magnetic disc.
As the results of various investigations on magnetic discs of this type, the inventors have discovered that a large problem exists in such magnetic discs. That is, when a magnetic disc is produced by simply attaching a flexible disc to each surface of a base plate as shown in FIG. 1, a so-called modulation, i.e. the ratio of the minimum output and the maximum output in one rotation of the magnetic disc becomes about 0.4 (as a matter of course, the ideal value of the modulation is 1 and it is better that the above-described ratio is as near to 1 as possible for best result). Accordingly, it is required to further improve the magnetic characteristics of magnetic discs of this type in order to put such to practical use.